In a stereotypical PC RPG, the player often has the freedom to progress as he prefers, usually in a number of skills or abilities with a level cap. It also has quests, where the player gets experience in a determined skill or some kind of special item. Most PC RPG's have a main storyline, much like the games in the adventure genre. Items are obtained by training a determined skill, doing quests, trading with NPC's, etc... Non-Player Characters (NPC) help the player or destroy him. The player usually wears some kind of armour and weapon (taking into account a combat triangle: melee, ranged and magic). The world changes as the player progresses.
Another requirement for a game to be a PC RPG is, logically, that it should be played in a computer. These only differ with console RPG's in the controls (usually).